Yom Tov falls on Sunday night. Just before one is about to make Kiddush, one reminds himself that he forgot to make Havdalah on Saturday night. What should he do? Say them together, first Kiddush, then Havdalah?
Another question: Tisha B’av falls on Monday night. During the seuda hamafsekes, one reminds himself that he still needs to say Havdalah, and the only beverage he has that is valid for Havdalah is wine. What should he do? (cannot give it to a katan)
Say kiddush on one cup and havdalah on another.
I would say havdalah on wine as only if it is possible to give to a child is it preferable. Also the Chazon Ish and others used to make havdalah on wine Motzei Shabbos Chazon.
O:CH 299 Magen Avrohom 9, O:CH 298:6, Biur Halacha ד”ה ולקבל , O:CH 551:10 Ramo MIshnah Berurah 68Dinim Vehanhokos Chazon Ish O:CH 10:15, Hilchos Tisha Beav 19:8.